The present invention relates to a negative pressure booster equipment to be used for brake booster equipment.
In the brake booster equipment using negative pressure, a tandem type brake booster equipment having two power pistons in tandem has been used conventionally as described, for example, in the Japanese Provisional Utility Model Publication Sho 63-53860 in order to obtain high braking power by lower brake pedal depressing force.
In such tandem type brake booster equipment, the control valve is switched over and the atmospheric air is introduced into two variable pressure chambers divided by two power pistons when brake pedal is depressed for braking operation, and by the action of the air thus introduced, two power pistons are operated. By the action of these power pistons, the master cylinder is operated, and brake fluid pressure is generated, resulting in the braking operation. Because the piston of master cylinder is operated by two power pistons in this case, the maximum braking fluid pressure is increased more than that of the brake booster equipment having a single power piston.
In the valve body to accommodate the control valve of such conventional type brake booster equipment, the cylindrical portion with rear shell cannot have very large diameter, whereas the control valve is installed just in the cylindrical portion. On the other hand, the valve body is provided with a vacuum passage communicating the control valve with the constant pressure chamber, and it was impossible in the conventional type equipment to enlarge the sectional area of the end portion of the vacuum passage on the control valve side because of the shape of the valve body.
For this reason, in case the air in the variable pressure chamber flows out toward the constant pressure chamber through the control valve and the vacuum passage when the operation of booster equipment is cancelled, the air is throttled at the end of the vacuum passage on the control valve side. When the air is throttled in this way, the return of the power piston of the booster equipment is delayed, and the release of the brake is delayed. In the tandem type booster equipment as described above, a large quantity of the air in two variable pressure chambers must pass through the portion with smaller sectional area, and the return of the power piston is extremely delayed.
Thus, there is the need to enlarge the sectional area of the end of vacuum passage on the control valve side. However, mere enlargement of the sectional area causes another problem, i.e. the increase in the size of the booster equipment.